1. Field of the Invention
The present invention relates to a cell driving type piezoelectric actuator comprising a plurality of cells each being defined by wall portions, and capable of achieving a predetermined function due to the volume change of the cell(s) by displacement caused by a piezoelectric operative portion provided on the wall portions.
2. Description of the Related Art
It has passed a long time since an ink jet head was employed for a printer, facsimile machine, copying machine, and other printing apparatuses. In recent years, a non-impact system has been used especially in most of small-sized printers, and a clear image has been reproduced on paper by the sophisticated ink jet head as in a silver salt photograph.
A printing apparatus of the non-impact system means an apparatus for printing by discharging an ink via a nozzle to print some images and/or characters to be printed on a printing medium (mainly paper). They are classified mainly into a piezoelectric system and a thermal jet system (Bubble Jet (registered trademark), etc.), depending upon type of the ink jet head discharging an ink. In these systems, the piezoelectric system denotes a printing apparatus in which a piezoelectric actuator is used as an ink jet head. This ink jet head mainly comprises a nozzle, an ink chamber communicating with an ink supply path, and a piezoelectric device capable of causing a volume change in the ink chamber. In the printing apparatus of the piezoelectric system, the ink jet head carries out typing or printing by causing the volume change in the ink chamber while applying a driving voltage to the piezoelectric device to discharge an ink from the nozzle onto a printing medium. Since the ink jet head of the piezoelectric system is not required to heat an ink like the thermal jet system, this type of the printing head has such advantages that the freedom in the selection of the ink is high, and that the controllability of the printing performance is excellent.
On the other hand, there has been a limitless demand for realization of a clearer and faster printing apparatus, and the ink jet head of the piezoelectric system has also been requested to further enhance an ink discharge power and an arrangement density of the piezoelectric device and ink chamber. Therefore, many proposals have heretofore been made to meet these requests. For example, an ink jet head having a high discharge power has been disclosed in the Patent Document 1 (See JP-A-6-143564). As shown in FIG. 1 of this Document, the ink jet head disclosed therein comprises plural number of chambers wherein every other chamber (channel) mutually separated by fine piezoelectric splits is filled with an ink to form an ink chamber (ink channel), and wherein a dummy chamber (dummy channel) is disposed between the ink chambers. An ink in the ink chamber is discharged through a nozzle by deforming the fine piezoelectric splits on both sides of the ink chamber by a contraction/expansion mode. In the ink jet head, the fine piezoelectric splits are polarized toward the individual chambers, an electrode is formed on at least the surface facing to the individual chamber (side surface) of each fine piezoelectric splits so as to constitute a piezoelectric device, and a driving voltage having a polarity in accordance with the polarized direction of the fine piezoelectric splits is applied to each electrode of the fine piezoelectric splits on the both sides of the ink chamber so as to displace the fine piezoelectric splits, and the ink chamber is contracted/expanded to change the volume of the chamber as a result, thereby an ink is discharged. When the height of the fine piezoelectric split is increased, the volume change of the ink chamber is increased without raising the driving voltage or increasing a width of the fine piezoelectric fine split which would result in lowering of the arrangement density, and the ink discharge power is enhanced.
Further Patent Document 2 (See Japanese Patent No. 3217006) has disclosed an ink jet (recording) head capable of solving problems of the ink discharge power and crosstalk. As shown in FIG. 1 of this Patent Specification, the proposed ink jet head is a one wherein the electrode coated with a protector layer is disposed on the inner surface of a piezoelectric substrate (plate) to form a groove covered with a lid (top plate) on an upper side. The ink chamber (ink channel) charged with the ink or the dummy chamber (dummy channel) are formed every other groove, and a nozzle plate including the nozzle communicating with the ink chamber is disposed. The both side walls of piezoelectric body defining the ink chamber i.e., the piezoelectric devices wherein the electrode is formed are displaced and deformed to discharge the ink from the nozzle in the ink jet head. For the ink jet head, the lid includes a slit extending through the nozzle plate above the dummy chamber, a depth of the dummy chamber is larger than that of the ink chamber, and the dummy chamber has a depth to such an extent that slip deformation of a bottom part of the dummy chamber is not caused by the deformation of the piezoelectric side walls constituting the ink chamber. Therefore, the crosstalk is reduced.
However, the above-described conventional ink jet heads are supposed to have several problems. For the ink jet head of Patent Document 1, a first problem is generation of crosstalk. As shown in FIG. 1 of Patent Document 1, since all the fine piezoelectric splits are bound by the upper lid and lower insulating support substrate in the described ink jet head, the contraction/expansion of one fine piezoelectric split would cause the deformation of another fine piezoelectric split through the lid and insulating support substrate, and the crosstalk is supposed to be generated. As a result, a diameter of an ink droplet discharged from the ink chamber or a discharge speed becomes nonuniform, and enhancement of quality of typing/printing is possibly limited.
Moreover, a embodiment in which all the fine piezoelectric splits are bound by the upper lid and lower insulating support substrate is a factor which obstructs the displacement of the fine piezoelectric splits defining the ink chamber. This results in a second problem that the enhancement of displacement efficiency is limited. The displacement efficiency can be represented, for example, by displacement/driving voltage of the piezoelectric body. That is, a ratio of displacement in the piezoelectric body relative to the driving voltage is indicated. When the displacement of the piezoelectric body increases, the volume change of the ink chamber also increases. In the case of the device disclosed in Patent Document 1, when the height of the fine piezoelectric splits is increased, the volume change of the ink chamber is increased without raising the driving voltage. However, since the displacement generated by the fine piezoelectric split is obstructed due to the structural limitation derived from the lid and insulating support substrate, it is supposedly difficult to increase the displacement per driving voltage.
A third problem is the drop of driving reliability with a demand for densification of the ink jet head in recent years. When a conductive liquid is used as an ink, it is indispensable to form the protector film in order to insulate the electrode. Furthermore, depending on a driving polarity, it is necessary to form the protector film in order to prevent electrolysis of the ink regardless of whether or not the ink has conductivity. A manufacturing method is not concretely described in Patent Document 1. However, when sputtering, chemical vapor deposition (CVD) or the like is used as means for forming the protector film, it would become difficult to form a unified, continuous, and highly reliable protector film because of surface tension of the material of the protector film with the narrowing the width of ink chambers and dummy chambers formed between the fine piezoelectric splits. This is because the narrower the spaces between them would result in the formation of closed spaces if the narrower the interval between the fine piezoelectric splits is made in order to raise the density. Then, problems such as short-circuit of the electrode are easily caused, and the driving reliability is lowered. For the forming of the electrode, because of the surface tension of the material of the protector film, it is difficult to form the uniform, homogeneous and highly reliable protector film for a similar reason, and there is possibility that conductive defect of the electrode is caused and the driving reliability is also lowered.
A fourth problem is a drop of a mounting property because of the demand for the densification with respect to the ink jet head in recent years. More concretely, the mounting property denotes a positioning precision or certainty during the connecting or bonding of a driving circuit or a power supply circuit to the ink jet head. Even when the ink jet head itself is capable of enhancing the discharge power and arranging the ink chambers or the piezoelectric devices in a high density and enhancing resolution, printing speed and the like, it is difficult to secure the high reliability over a long time if the mounting property is deteriorated.
The first to fourth problems of the ink jet head according to Patent Document 1 is also applicable to the ink jet (recording) head according to Patent Document 2 in consideration of the resolution and printing speed required for the printing apparatus in recent years. Needless to say, the ink jet head described in Patent Document 2 is supposed to be improved in the crosstalk (first problem) and displacement efficiency (second problem), compared with the ink jet head disclosed in Patent Document 1. This is because the lid includes the slit extending through the nozzle plate above the dummy chamber, the depth of the dummy chamber is larger than that of the ink chamber, and the dummy chamber is deep to such an extent that the slip deformation of the bottom part of the dummy chamber is not caused by the deformation of the piezoelectric body side walls constituting the ink chamber. However, the grooves are formed in the piezoelectric body substrate to constitute the ink and dummy chambers, and the piezoelectric body side walls which are basically displacement generating portions are connected to each other via the bottom part. Therefore, in the embodiment in which the arrangement density is enhanced to enhance the resolution and a plurality of piezoelectric body side walls are disposed in the vicinity of one another, effects of the reduction of the crosstalk and the enhancement of the displacement efficiency do not supposedly have a satisfactory level. It is noted that in case of the ink jet head according to Patent Document 2, the third and fourth problems are supposed to be remained unsolved since there is no mentioning as to these problems therein.